A. Field of Invention
This invention pertains to the art of methods and apparatuses regarding the determination of the coefficient of friction of a composition, especially an elastomeric material such as the tread of an existing tire.
B. Description of the Related Art
Coefficient of friction is an intrinsic property of a material and the substrate it is touching. Therefore, its value should be geometry independent. In practice, however, this is typically not the case. It is well know, for example, to determine the “coefficient of friction” of a tire tread in a manner where the geometry of the tread blocks and tread sipes within the blocks affect the result. It would be more accurate to refer this property as “coefficient of traction” because sample geometry is a contributing factor.
It is desirable to describe coefficient of friction in terms of contact stress (pressure), sliding velocity or strain rate, temperature, humidity, substrate properties, etc. With reference to FIG. 1, conventional tests for coefficient of friction often use a block specimen 1 where the specimen's geometry contributes to the frictional force (Fxy) developed between the specimen 1 and the substrate 2. This is due to mechanical interference between the leading edge 3 of the block 1 and the substrate 2 and the non-uniform distribution of contact stresses at the interface. A phenomenon known as “frictional lift” often occurs as the trailing edge 4 of the block 1 loses contact with the substrate 2 because of the couple induced in the specimen 1. These problems at the leading edge 3 and the trailing edge 4 of the specimen 1 are, in this patent, referred to individually and collectively as “edge effects.” The goal for the measurement of coefficient of friction is therefore to minimize geometric effects so as to best approximate the true point-wise value
U.S. Pat. No. 6,349,587, titled PORTABLE UNIVERSAL FRICTION TESTING MACHINE AND METHOD (the '587 patent) is an example of a conventional friction testing machine used with tire compounds. This machine is said to measure the coefficient of friction between a rubber specimen and a friction surface. It should be pointed out, however, that the friction testing machine of the '587 patent as well as most of those known in the prior art do not account for the specific geometry of the test samples and thereby edge effects distort the true friction characteristics.
U.S. Pat. No. 5,113,688, titled LABORATORY TRACTION TEST (the '688 patent) has a common assignee and is incorporated herein by reference. The '688 patent discloses an apparatus, called a Rotational Friction Tester (RFT), that predicts tire traction characteristics by measuring the friction characteristics of the samples. The specific geometry of the samples, shown in FIG. 3B of the '688 patent, eliminates the edge effects to better isolate the actual friction characteristics of the sample. These samples, however, must be carefully formed and cured into this particular shape in order to remove the edge effects. As a result, it is not possible to find accurate friction characteristics of samples from existing tires or other products where the specific composition of the product is unknown.
This invention solves these problems by providing a friction testing apparatus and method that eliminates edge effects and provides accurate coefficient of friction measurements for samples independent of the specific sample geometry.